But the super PAC's ad buy marks a shift in the race for former Rep. Jesse L. Jackson Jr.'s seat on Chicago's south side. It's possible none of the candidates, including Halvorson, will raise enough money to air campaign ads in the Windy City's pricey market. So far, the better-funded campaigns have relied on direct mail to reach voters.

Halvorson boasted about the National Rifle Association's endorsement when she represented a much more conservative district nearby. But it's become a liability in this crowded race for the heavily Democratic district.

The large 2nd district special election field includes Alderman Anthony Beale, former NFL linebacker Napoleon Harris, state Sen. Toi Hutchinson, Cook County Chief Administrative Officer Robin Kelly, former Rep. Mel Reynolds and Halvorson. The primary is scheduled for Feb. 26.

The spot will air on both cable and broadcast networks, according to a PAC spokesman, who declined to give the exact size of the buy.